The invention relates to phase encoding methods which enable the transmission of multiple PSK modulated signals and more particularly the transmission of differential and non-differential phase modulated signals on a single channel.
In the ANSI 136 Rev. 0 standard which is an existing standard for TDMA (time division multiple access) communications, there is a DTC (Digital Traffic Channel) slot format specified which supports speech using only VSELP (vector-sum excited linear predictive) coding and IS-641 ACELP (algebraic code excited linear predictive) advanced 8-bit vocoders with xcfx80/4-shifted DQPSK (differential quadrature phase shift keying). The DTC slots are transmitted as part of a six slot frame which is up converted to a particular carrier. In slot formats such as these which are strictly differentially encoded, at a mobile receiver, channel parameters which are estimated during the reception of the SYNC (synchronization) sequence can be used to decode the rest of the slot because there is no phase discontinuity.
ANSI-136 Rev. A is a newer version of the standard which is backwards compatible with ANSI-136 Rev. 0 so as to support the above described xcfx80/4-shifted DQPSK encoded slot format. In addition, it also supports new DTC slot formats which feature US1 vocoder and 8-PSK modulation. This downlink DTC slot format begins with a SYNC sequence which is xcfx80/4-shifted DQPSK encoded followed by the remainder of the slot which is 8-PSK encoded.
During the SYNC sequence, the differentially encoded phases are shifted by an initial phase. During the remainder of the slot where 8-PSK is employed, absolute phase values are transmitted. In the presence of a non-zero initial phase, this results in a phase discontinuity at the boundary between the SYNC sequence and the remainder of the slot, and this phase discontinuity can result in various performance degradations. For example, the channel information estimated during reception of the differentially encoded SYNC sequence (in the presence of the initial phase) cannot be used to decode the remaining non-differentially encoded phases which do not include the initial phase.
In an existing proposal for handling this phase discontinuity, a reference symbol, REF, is inserted into the 8-PSK downlink slot structure immediately after the SYNC sequence to enable the determination of a carrier phase reference for the remaining 8-PSK symbols. While this reference symbol does solve the problem, it takes three bits (one phase) to implement, and is therefore a waste of precious bandwidth. Furthermore, in the presence of frequency selective fading, one reference symbol may not be enough to recover sufficient channel information to accurately decode the remaining data.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
A preferred embodiment of the invention provides a phase encoding method for switching between DPSK (differential phase shift keying) and N-PSK (N-ary phase shift keying) which maintains phase continuity and which does not require any reference symbols when using N-PSK. A sequence of known DPSK phases is encoded with respect to an initial phase. The N-PSK phases are then all offset by this same initial phase.
An encoder, basestation and mobile station for implementing embodiments of the invention are provided. Advantageously, by eliminating the need for a reference symbol when using N-PSK, log2N additional bits are available for information or error correction coding.
The method is more generally applicable to any situation where a switch between a phase encoding method which requires an absolute phase reference and a phase encoding method which does not have an absolute phase reference is to be implemented. More generally, it can also be applied when there is a known sequence which is preceded by an unknown sequence, followed by an unknown sequence, or preceded and followed by unknown sequences.